The Transport Layer

Underlying any messaging system is the transport or wire protocol that
governs the serialization of the message as it is sent across a wire and the
interpretation of the message bits when it gets to the other side. Although
SOAP messages can be sent using any number of protocols, the SOAP specification
defines only the binding with HTTP. SOAP uses the HTTP request/response message
model. It provides SOAP request parameters in an HTTP request and SOAP response
parameters in an HTTP response. The HTTP binding has the advantage of allowing
SOAP messages to go through firewalls.